1. Field of the Invention
The present invention relates generally to railway rolling stock, and in particular to a system for piggybacking three semi-trailers on two modified flat cars.
2. Description of the Prior Art
In the transportation industry, it is often more economical to transport semi-trailers "piggyback" on railway flat cars than it is to pull them with tractor units over the road. The cost advantage enjoyed by railway transportation in this regard is normally proportional to the distance involved because the terminal costs associated with loading and unloading the trailers on flat cars are less significant in comparison to the total transportation costs on long routes. Accordingly, there have been developed various prior art devices for securing semi-trailers on railway flat cars. For example, the Fahland U.S. Pat. No. 2,916,238 and the Smith U.S. Pat. No. 3,358,954 both show foldable fifth wheel stanchions for mounting on railway flat cars and for releasable attachment to the king pin of a semi-trailer.
However, it has been recognized that there are inefficiencies inherent in the basic concept of transporting semi-trailers on railway flat cars. For example, the existing flat car rolling stock used by many railroads for piggyback transportation tends to be relatively heavy and designed for far greater load capacities than are normally incurred in transporting semi-trailers. Thus, a railroad incurs a substantial weight penalty in piggyback operations which in turn leads to higher fuel costs per unit of travel distance. The Grandpre U.S. Pat. No. 3,033,129 and the Kirwan U.S. Pat. No. 4,179,997 each addresses this problem and provides for a transport system wherein much of the bulk associated with normal railway flat cars is eliminated. However, the systems proposed in the Grandpre and the Kirwan patents require rolling stock designed specifically for piggyback operation, and cannot be accomplished through practical modification of existing railway rolling stock.
Another problem with piggyback service relates to matching the lengths of the trailers to the flat cars. It is this problem to which the present invention is addressed. Although flat cars are available in lengths ranging from 40 feet to 90 feet and more, a fairly significant number are in the 80 to 90 feet range. Semi-trailers are also available in a variety of lengths, but a significant number are in the 40 to 50 feet range. Thus, it is fairly common that semi-trailers must be transported one-on-one because they are too long for double, tandem loading on available flat rolling stock. The Brodeur et al. U.S. Pat. No. 4,339,996 addresses this problem and calls for an articulated rail car unit having first and second car bodies each with its own truck units and adapted to receive a single semi-trailer. However, the cars proposed therein are specifically designed for the disclosed purpose and would not be readily convertible from existing railway stock. Thus, except when flat cars are matched with trailers either approximately equal to or approximately half their lengths, piggyback transportation of semi-trailers tends to be somewhat inefficient because relatively large amounts of space are wasted and unoccupied in one-on-one loadings.